The Classic Maya kept track of historical events using three main calendars. The Long Count represented the number of days ellapsed since the creation of the present Cosmos, which the Maya reckoned to have happened about five thousand years ago. This date was expressed in the vigesimal system, with units of 1, 20, 360, 7,200 and 144,000 days. Thus, a date of 9.8.9.13.0 (the birth of K'inich Janaab Pakal I of Palenque) translates to 1,357,074 days since the creation of the Maya universe.

In the Long Count app, you can change the date by pressing the + and - buttons of the corresponding unit on the upper glyph panel. You can also directly input the long count date on the form under the glyphs.

Besides the Long Count, the Classic Maya used two other calendars called Tzolk'in and Haab - which, together, form the Calendar Round. Unlike the Long Count, these calendars are cyclical, and a date in the Calendar Round will repeat itself every 52 years. The Tzolk'in is a 260-day cycle where each day has a number between 1 and 13 and a name in a sequence of 20 names. It starts with 1 Imix, 2 Ik, 3 Akbal ... until 13 Ben. Because there are more day names than numbers, the next Tzolk'in date will be 1 Ix and then continue until 7 Ahau. The cycle than resumes with 8 Imix, 9 Ik, 10 Akbal and so forth.

The Haab corresponds to the 365 days of the solar year and is divided into 18 'months' of 20 days (called Pop, Uo, Zip, Zotz ...), plus an additional month of 5 days (called Uayeb). The birth of Janaab Pakal, 9.8.9.13.0 in the Long Count, corresponds to a Calendar Round of 8 Ahau 13 Pop. When you change the Long Count date in the app, the Calendar Round is immediately updated. The date is also automatically converted to the Western calendar.

In order to perform the conversion, we need a correlation constant that anchors a Maya date in the Western calendar. Mayanists employ a Julian Day Number (a system used by astronomers) as a constant. This expresses the number of days ellapsed between the Julian Day Zero (Jan 1 4713 BCE) and the Maya Long Count 0.0.0.0.0. The most common correlation constant used in the literature is 584285, a little correction of Thompson-Goodman-Martinez correlation of 584283. However, more recently, Simon Martin and Joel Skidmore proposed a constant of 584286 that better aligns with astronomical events, and this is the default on the app. You can change that to any other correlation constant if you wish.

Maya dates are converted to the Gregorian Calendar and to the Julian Calendar. The Gregorian Calendar was only established in 1582. Before that, dates in the Western World were expressed using the Julian Calendar. Any Gregorian date before 1582 is called 'proleptic', because it is merely a projection. The difference between the two calendars is in the treatment of leap years. In the Gregorian Calendar, extra days are added to years exactly divisible by 400, which otherwise would not be leap years. Because this rule did not exist in the Julian Calendar, a difference of 10 days had accumulated between the Julian year and the actual solar year in 1582 when the Gregorian Calendar was instituted.

In the Long Count app, you can input a date in any of the Western Calendars and it will be immediately converted to the Maya Calendar depending on the correlation constant you choose.

Maya inscriptions connect events that happened on different dates using Distance Numbers. Distance numbers start to be counted from an initial Long Count date that appears at the beginning of the inscription. They represent the number of days to be counted forward or backward relative to the previous date.

For example, the ruler Janaab Pakal of Palenque whose birth we mentioned before acceded to the throne when he was only 12 years old. To be more precise, in the Maya vigesimal notation, his birth is separated from his accession by the distance number 12.9.8 or approximately 12 years (of 360 days), 9 months (of 20 days) and 8 days.

Maya inscriptions normally include only one Long Count date. The next dates, whether they are counted forward or backward, are introduced by distance numbers and only given in the Calendar Round format. Therefore, it is essential to keep track of distance numbers in order to reconstruct the corresponding Long Count - and, consequently, the equivalent date in the Western Calendar.

In the Distance Number app, you can enter a Long Count date and the Distance Number to the next date. The glyph panel is updated showing the respective Calendar Rounds. When you add those dates to the list at the bottom of the page, the Long Count is updated to the next position, so you can easily keep track of the sequence of dates in an inscription. For simplicity, the corresponding Western dates are given in the Julian Calendar using the 584286 correlation.